//GENERERA PLATSER
$result = $con->query("SELECT `seat` FROM `user_book`");

$seatsTaken = array();

if( mysql_num_rows( $result ) > 0 )
{
    while ( $row = mysql_fetch_assoc( $result ) )
    {
        $seatsTaken[] = $row['seat'];
    }
}

echo "<select name=\"seats\">";

for ($i = 1; $i <= 124; $i++)
{
    if( in_array($i, $seatsTaken ) )
        continue;

    echo "<option value=\"$i\">$i</option>";
}

echo "</select>";